In a magnetic disk drive, a head needs to be positioned at a target position on a disk (magnetic disk). Servo data (more specifically, servo burst data included in the servo data) is used for head positioning. The servo data is pre-written to servo areas discretely arranged at predetermined intervals in tracks (servo tracks) concentrically arranged on the disk. The servo areas with the servo data written thereto are referred to as servo frames.
To allow the head to be accurately positioned at the target position based on the servo burst data in the servo data, the center line of each of the tracks with the servo data written thereto is desirably shaped like a perfect circle. However, the shape of center line of each track on the disk is distorted and generally deviates from a perfect circle. This is mostly due to runout of a rotating shaft of a spindle motor that rotates the disk during servo data write (that is, the runout synchronizes with the rotation of the disk). If the shape of the center line of the track is distorted, the servo burst data involves errors resulting from the distortion, that is, errors resulting from the runout synchronizing with the rotation of the disk (what is called repeatable runout).
Thus, in recent magnetic disk drives, repeatable runout correction data is written to each servo frame. The repeatable runout correction data is digital data generated by encoding a correction amount required to correct (eliminate) an error caused by repeatable runout of the servo burst data (more specifically, the servo burst data read by the head and demodulated).
The demodulated servo burst data (that is, a demodulated value for the servo burst data) is corrected based on the correction amount indicated by the corresponding runout correction data. The correction eliminates the error caused by the repeatable runout from the demodulated value of the corrected servo burst data. Thus, the displacement of the head caused by the repeatable runout can be prevented by positioning the head at the target position based on the demodulated value of the corrected servo burst data. That is, the displacement of the head caused by the repeatable runout can be prevented by positioning the head based on the correction amount indicated by the repeatable runout correction data.
In the recent magnetic disk drives, the track pitch has been increasingly reduced in order to further improve recording density. It is assumed that an error occurs in such a magnetic disk drive when an attempt is made to position the head at the target position on the disk. In this case, even if the head (more specifically, a read element of the head) is slightly displaced from the target position of the target track (for example, the center line of the target track), the head approaches the boundary between the target track and the adjacent track. When thus located close to the track boundary, the head simultaneously reads the runout correction data from both the target track and the adjacent track.
Then, if different repeatable runout correction data have been written to the target track and the adjacent track, the repeatable runout correction data read from the target track is interfered with by the repeatable runout correction data read from the adjacent track. That is, crosstalk results from the inter-track interference. The crosstalk affects the read signal output by the head. Thus, if the read signal is demodulated, any of the relevant bits makes the demodulated value uncertain.
Thus, with the track pitch increasingly reduced, there has been a demand to prevent the interference of the repeatable runout correction data written to the adjacent track. There has also been a demand to maximally prevent a reduction in the size of each data area (that is, a reduction in track format efficiency) resulting from the write of the repeatable runout correction data to each track.